The invention relates to an apparatus which is suitable for being surrounded by molten glass and which may be designed, for example, in the form of an agitator, a needle, a plunger or rotary plunger or a Vello or down-draw needle, in order to be used in the glass industry, for example, for homogenizing molten glass. The invention relates, furthermore, to a method for processing molten glass, in which the apparatus is used.
Structural parts consisting of high-grade metal or high-grade metal alloys, such as preferably PGM materials, are employed in the glass industry, particularly in plants for the melting and hot forming of special glass. These plant components used in fusion technology, also called PGM (Platinum Group Metals) products, serve for melting, refining, transporting, homogenizing and portioning liquid glass.
Such structural parts, are essentially either structures consisting of solid PGM material or of materials resistant to high temperature, such as ceramic refractory materials or metallic special materials, with a thin-walled PGM cladding, for example, in the form of thin sheet metal or of a PGM surface coating which is applied, for example, by plasma spraying or flame spraying.
Plant parts carrying glass melt are often noble metal sheet structures which are designed as thin-walled pipe systems. The molten glass flows through these at temperatures of between 1000° C. and 1700° C. The pipe systems, as a rule, are surrounded on the outside by an insulating and, if appropriate, supporting ceramic, this, in turn, often being held by supporting metal structures, such as, for example, metal boxes.
Structural parts consisting of solid PGM material have molten glass flowing over or around them and are partly moved in the glass melt.
PGM (Platinum Group Metals) materials, because of their high melting point, are distinguished by high temperature resistance and, furthermore, by high mechanical strength and resistance to abrasion and are therefore especially suitable for the production of structural parts in plants or plant parts which come into contact with the glass melt. Suitable materials are platinum and alloys of platinum and/or of other PGM metals.
The prior art, as described in DE 43 26 143 A1, discloses a plunger which consists of a core consisting of molybdenum, tungsten or of an alloy of these. Furthermore, the plunger has a ceramic body which is provided with a covering consisting of platinum or of a platinum-rich alloy. The ceramic body serves for preventing intermetallic diffusion, specifically between the metal core and the platinum covering.
The plunger has the disadvantage, however, that the layer build-up with the molybdenum or tungsten core is relatively complicated. A further disadvantage is that, if the covering is damaged, oxidation of the molybdenum or tungsten core occurs, with the result that the plunger becomes useless.
Moreover, a plunger needle is known from U.S. Pat. No. 3,332,766, such a plunger needle in this case consisting of an outer cylinder with a hemispherical cap. Furthermore, reinforcing ribs are arranged inside the cylinder. Both the cylinder and the hemispherical cap and also the reinforcing ribs in this case consist of platinum or of a platinum alloy.